A fuller understanding of the neuronal organization of the visual pathways requires knowledge of how that organization varies between species with different life styles and phylogenetic histories. The most intensively studied species in visual neuroscience is the cat, a carnivore, and in recent years a powerful paradigm for understanding the topography of the cat's retina, and the neuronal organization of the cat's visual pathway has been developed which turns on a functional classification of cat retinal ganglion cell into three broad groups, the so-called, Y-, X- and W-cells. We propose to extend these studies to selected other species with distinct phylogenetic histories, including the human, monkey and the bushbaby (all primates), the rabbit (a lagomorph), and marsupials such as the Australian possum and wallaby. We seek to determine what components of the Y/X/W paradigm are common to quite different mammals and which are specific to particular species. It is proposed to study the pattern of distribution of ganglion cells in retinal whole mounts of each species, the soma size spectrum of ganglion cells at various retinal locations (central specialized area, visual streak, periphery), and to trace the projections of ganglion cells to brain nuclei (e.g. lateral geniculate nucleus and superior colliculus). With physiological techniques, we will attempt a classification of ganglion cells according to the axonal conduction velocity, receptive field properties and central projections of a surveyed sample of retinal ganglion cells. The physiological analysis will be extended to the visual relay nucleus of the thalamus (the lateral geniculate nucleus) to test whether, as in the cat, its organization is strongly influenced by its retinal input. In the monkey, for which the basic geniculate study has been done, we propose to analyse the visual cortex in terms of the input it receives from different classes of geniculate relay cell. By studying the human pathway in the context of intensive work on several animal species we will be able to make a more incisive, better-directed analysis of human vision as well as a broad and integrated survey of the organization of mammalian visual pathways.